Connected: An Internet Encyclopedia
RFC 1521 - 7.3. The Message Content-Type

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### 7.3. The Message Content-Type

It is frequently desirable, in sending mail, to encapsulate another mail message. For this common operation, a special Content-Type, "message", is defined. The primary subtype, message/rfc822, has no required parameters in the Content-Type field. Additional subtypes, "partial" and "External-body", do have required parameters. These subtypes are explained below.

          NOTE: It has been suggested that subtypes of message might be
          defined for forwarded or rejected messages.  However, forwarded
          and rejected messages can be handled as multipart messages in
          which the first part contains any control or descriptive
          information, and a second part, of type message/rfc822, is the
          forwarded or rejected message.  Composing rejection and forwarding
          messages in this manner will preserve the type information on the
          original message and allow it to be correctly presented to the
          recipient, and hence is strongly encouraged.

As stated in the definition of the Content-Transfer-Encoding field, no encoding other than "7bit", "8bit", or "binary" is permitted for messages or parts of type "message". Even stronger restrictions apply to the subtypes "message/partial" and "message/external-body", as specified below. The message header fields are always US-ASCII in any case, and data within the body can still be encoded, in which case the Content-Transfer-Encoding header field in the encapsulated message will reflect this. Non-ASCII text in the headers of an encapsulated message can be specified using the mechanisms described in [RFC-1522].

Mail gateways, relays, and other mail handling agents are commonly known to alter the top-level header of an RFC 822 message. In particular, they frequently add, remove, or reorder header fields. Such alterations are explicitly forbidden for the encapsulated headers embedded in the bodies of messages of type "message."

7.3.1. The Message/rfc822 (primary) subtype
7.3.2. The Message/Partial subtype
7.3.3. The Message/External-Body subtype

7.3.3.1. The "ftp" and "tftp" access-types
7.3.3.2. The "anon-ftp" access-type
7.3.3.3. The "local-file" and "afs" access-types
7.3.3.4. The "mail-server" access-type
7.3.3.5. Examples and Further Explanations

Next: 7.3.1. The Message/rfc822 (primary) subtype

Connected: An Internet Encyclopedia
RFC 1521 - 7.3. The Message Content-Type